Sailors have taken part in gay pride parades for years, alongside cowboys, transvestites and plenty of men in leather chaps.

But today is the first time genuine members of the Royal Navy will march openly in full uniform, at this year's Europride parade, hosted by the City of London.

Just like a military parade, there will be a marching order. First come the Mayor of London, dignitaries including Sir Ian McKellen and the organisers. Next come the disabled marchers, and then the sailors, real ones - represented by a group of about 30 gay, lesbian and bisexual navy personnel wearing their "number one" uniform of formal blues, with medals pinned to breasts.

Leading the group will be Chief Petty Officer Mark Probee, who joined the navy 20 years ago. Like many young men, he was attracted to military service as an opportunity to learn a trade and travel with it. He had been working at a desk job and wanted something more "thrilling". But unlike other young recruits, he joined knowing his sexuality was a sackable offence.

"I knew it was something I would have to hide," he said. "For a lot of years I lost out on that, not being able to love someone openly and freely. It might have made me emotionally a stronger person because all that time I was having to privately deal with something other people can do without thinking."

Probee says he has never "flaunted" his sexuality, but he was lucky it was not exposed before the ban on homosexuals in the armed forces was lifted in 2000.

Others were not so fortunate: father of two Graeme Grady was discharged from the RAF in 1994 after attending a counselling group for married gay men, and John Beckett, a weapons engineering mechanic on the nuclear submarine HMS Valiant, was discharged in 1993 after he told his commanding officer and his chaplain about his gay relationship. Jeanette Smith was an RAF nurse when an anonymous informant told her superiors she was in a lesbian relationship, and Duncan Lustig-Prean was a naval lieutenant commander when he was discharged in 1994 after reporting a blackmail attempt over his sexuality.

In 1998 the gay rights group Stonewall took up these four cases and mounted a legal challenge to the armed forces' "no gays" policy. At the time there was no Human Rights Act and although judges in the high court and the court of appeal said the ban was not justified, they had no jurisdiction to overturn it.

The fight went all the way to the European court, where the case succeeded, forcing the Labour government to lift the ban. The armed forces introduced a new code of sexual conduct in 2000.

Stonewall spokesman Alan Wardle said the navy had undergone an "amazing transformation" since then. "It was less than 10 years ago that [gay] people were sacked and subjected to humiliating cross-examinations and things."

The gay, lesbian and bisexual naval personnel will join the estimated 50,000 others expected to turn up today. Organisers estimate the entire crowd will swell to 500,000. The parade will follow a route down Oxford and Regent Streets, before filing past Trafalgar Square and the Houses of Parliament and finally dispersing at Victoria Embankment.

CPO Probee had never attended a pride event before, let alone marched in one. "I always thought, 'Why do I want to march down the street being proud of my sexuality when straight people don't?' But it's a bit of a different reason this time.